Playdates should be informal get togethers for moms and kids. I think a written note, although a nice gesture, may be a bit over the top. I think even bringing a pie was a bit formal. Not saying you are not nice to do that. I am sure she appreciated it and everyone enjoyed it but playdates are usually easy going. However since she ordered pizza then I guess the pie was a good thing to bring. For us usually playdates are bring your own kid's and your own food and just hang out playing and chatting. I would just email her a "had a great time" email.

I would mail it. Written correspondence is a dying art form. Just think how nice she will feel when she gets a nice ty note in the mail. And bringing the pie was a smart move. No matter where I go, I bring a dish and send a thank you note, no matter what. I am old school.